fixes #6594 - adding content host docs

That doesn't make any sense, are you proposing we add something in addition to that?

```
+### How is subscription-manager installed ?
+
+* RHEL, subscription-manager is installed by default
+* Fedora, subscription-manager is available from the Everything repo for its release (a simple 'yum install subscription-manager' will suffice)
+* CentOS, enable the upstream subscription-manager repo and then install subscription-manager:


``